Key Considerations for Selecting Medium-Voltage Vacuum Circuit Breakers

The first step in selection is to confirm whether the circuit breaker can be used in the system.

The rated voltage must be higher than the system's highest operating voltage-12kV for a 10kV system, 24kV for a 20kV system, and 40.5kV for a 35kV system.

This is the safety margin stipulated by national standards; the system voltage cannot be treated as the equipment voltage.

The rated current must be greater than the circuit's maximum continuous operating current, while also considering long-term load growth. Common rated currents include 630A, 1250A, 1600A, 2000A, 2500A, 3150A, and 4000A. For example, a 2000kVA transformer has a rated current of approximately 115A on the 10kV side.

However, considering ring networks and overload, a 630A rating is generally chosen. If future capacity expansion is needed, a 1250A rating can be selected directly.

This is the most crucial performance indicator of a circuit breaker, determining its ability to withstand a short circuit.

The rated short-circuit breaking current must be greater than or equal to the maximum expected short-circuit current at the installation point. Common ratings are 20kA, 25kA, 31.5kA, 40kA, and 50kA.

For urban substations with large short-circuit capacities, a rating of 31.5kA or higher is recommended; for terminal distribution circuits, 25kA is usually sufficient.

Another easily overlooked parameter is the rated short-circuit making current (peak value), which is typically 2.5 times the breaking current.

It assesses the mechanical strength of the circuit breaker when it encounters a short circuit during closing-if this indicator is insufficient, the circuit breaker may explode at the moment of closing.

The environment in which a circuit breaker is installed directly affects its service life.

Standard products are suitable for altitudes below 1000 meters. Above this altitude, the air is thin, insulation strength decreases, requiring either derating or the selection of a "high-altitude" type.

Polluted environments are also critical-in chemical plants, coastal areas, and dusty locations, ordinary insulators are prone to creepage and flashover. In such cases, it is recommended to choose a solid-insulated pole type, where the arc-extinguishing chamber is entirely cast in epoxy resin, providing protection against pollution, moisture, and corrosion, making it far more reliable than traditional insulated cylinder types.

Furthermore, it's important to distinguish between indoor and outdoor installations: indoor types installed in switchgear only require an IP4X protection rating; outdoor pole-mounted types must use pole-mounted vacuum circuit breakers with metal casings, an IP54 or higher protection rating, and the ability to withstand sun and rain.

This part determines whether the circuit breaker can operate reliably. Mechanical lifespan is a crucial indicator.

While 10,000 cycles are sufficient for ordinary power distribution, in environments like steel mills and rolling mills where operations are repeated dozens of times daily, ordinary circuit breakers can be worn out in just a few years.

Frequent operation types with a mechanical lifespan of 30,000 cycles or more must be selected.

The type of operating mechanism also matters: spring mechanisms are the most common, allowing manual operation even during power outages, and are also affordable; permanent magnet mechanisms are simple in structure, maintenance-free, and have a long lifespan, suitable for frequent operation, but manual operation is cumbersome when power is off.

The control voltage must match the control system. Commonly used voltages are DC110V, DC220V, and AC220V.

This must be specified when ordering, otherwise, the circuit breaker may not close on-site.

The vacuum circuit breakers are not isolated; they must be installed in switchgear, and the dimensions and interfaces must fit perfectly.

First, it's essential to distinguish between truck-mounted and fixed types-KYN28 medium-voltage switchgear can only be paired with truck-mounted circuit breakers, such as VS1-12; XGN or HXGN fixed switchgear is paired with fixed type circuit breakers.

Retrofitting older circuit breakers is more complicated, requiring verification of phase-to-phase distances, mounting hole spacing, and contact box dimensions.

Ideally, photos of the site should be taken to confirm whether our products can be directly replaced.

Many retrofit projects have to be reworked due to dimensional incompatibility.

Different industries have additional requirements for circuit breakers.

The short-circuit current at the generator outlet decays slowly and contains a high DC component;

ordinary circuit breakers may not be able to open it, requiring a specialized generator outlet circuit breaker.

Switching capacitor banks requires interrupting capacitive current, necessitating the selection of C2-class (capacitive breaking capacity) products; otherwise, reignition and overvoltage are likely.

High-altitude and frigid regions also require special handling-high-altitude areas require high-temperature type circuit breakers with improved insulation levels; in northern winters with temperatures dropping to -30 or -40 degrees Celsius, heaters are needed inside the mechanism to prevent freezing.

These details are easily overlooked in general selection, but they are precisely the areas most prone to later failures.